Resin transfer molding process

ABSTRACT

A wind tunnel blade ( 30 ) connected to a base ( 32 ) and held in position by a two-piece cuff ( 34 ). The wind tunnel blade ( 30 ) is formed in a resin transfer molding process in which central, fore, and alt foam core sections ( 70, 72, 74 ) are placed together to form the wind tunnel blade ( 30 ). Radius fillers ( 120 ) are used to fill the gaps between the outer edge of the foam core sections. The radius fillers ( 120 ) used in the wind tunnel blade ( 30 ) are formed by a braided sleeve ( 122 ) surrounding a number of unidirectional tows ( 124 ). A tip ( 68 ) is formed separately from the rest of the wind tunnel blade ( 30 ) and is glued to the top thereof. Stacked layers of braided fibers ( 100 ) are used to reinforce the central core section ( 70 ).

FIELD OF THE INVENTION

[0001] This invention relates to a process of resin transfer moldinglightweight, foam-filled products and the strong, lightweight productsmade thereby. More particularly, the present invention is directed to aprocess of resin transfer molding a wind tunnel blade and the structureof the wind tunnel blade.

BACKGROUND OF THE INVENTION

[0002] Resin transfer molding has been around for many decades, and itsuse has grown considerably in recent years. The process allows theeconomical manufacture of high quality composites. In accordance withthe process, a resin system is transferred at low viscosities and lowpressures into a closed mold die containing a preform of dry fibers. Thedry fibers, which may have the form of continuous strand mat,unidirectional, woven, or knitted preforms, are placed in a closed moldand resin is introduced into the mold under external pressure or vacuum.The resin cures under the action of its own exotherm, or heat can beapplied to the mold to complete the curing process.

[0003] The resin transfer molding process can be used to producelow-cost composite parts that are complex in shape. These partstypically provide continuous fiber reinforcement, along with inside moldline and outside mold line controlled surfaces. It is the placement ofthe continuous fiber reinforcements in large structures that sets resintransfer molding apart from other liquid molding processes.

[0004] In the past, resin transfer molding was used for applicationssuitable to consumer product markets. However, in the last few years,through the development of high-strength resin systems and more advancedpumping systems, resin transfer molding has advanced to new levels.These recent developments have promoted resin transfer moldingtechnology as a practical manufacturing option for high-strengthcomposite designs, particularly in the aerospace industry.

[0005] In the aerospace industry, the most visible advantage to theresin transfer molding process lies in resin transfer molding's abilityto combine multiple, detailed components into one configuration. Forexample, many traditional designs consist of many individual detailsthat are combined as a subassembly. These subassemblies usually requirelabor-intensive shimming, bonding, mechanical fastening, and sealing.Consequently, these subassemblies demonstrate high part-to-partvariability due to tolerance build-up.

[0006] Resin transfer molding produces an aerodynamic, decorativefinish, with controlled fit-up surfaces. Being a product of the moldmakes the surface quality of the part produced within the moldcomparable to that of the tool's surface.

[0007] Resin transfer molding also provides control of the fiber/resinratio in the completed product. This advantage produces parts that arelightweight and high in strength.

[0008] Unlike conventional composite systems that use lay-up of prepregmaterials, resin transfer molding does not require an autoclave.Therefore, no autoclave costs are incurred, no size limitations areinherent, and no staging issues occur.

[0009] In terms of raw material cost, resin transfer molding offers costsavings by using bulk materials like broad goods. Because dry goods areless expensive than preimpregnated materials, savings can be associatedwith the cost of the wasted material during the ply-knitting operation.Also, bulk materials do not require special handling requirements suchas freezer storage.

[0010] The basic injection operation of resin transfer molding isstraightforward and easily learned. Hence, minimal training is requiredto bring operators on line. On the other hand, in making preforms, thelevel of operator skill and training is dependent upon the method ofpreforming that is used. Preform fabrication methods include braiding,knitting, weaving, filament winding, and stitching. Each of thesemethods is quite different and must be individually evaluated forspecific design characteristics.

[0011] The initial capital investment costs of resin transfer moldingare low when compared with many other molding processes. An elementaryform of resin transfer molding can be achieved using a pressure pot, anoven, and a vacuum source. A variety of commercially available equipmentcan be used to enhance the process in many areas.

[0012] In most cases, resin transfer molded materials can be formed withminimal chemical exposure to workers and their environment. Manyhigh-performance resin systems are stable and release low volatiles.Since resin transfer molding is processed within a closed system,workers are exposed to the resin only when loading the dispensingequipment.

[0013] One of the problems encountered when using resin transfer moldingis that complex cavities that extend into the surface of the part mustbe formed in the mold cavity surface, or the complex cavity will befilled by resin during the resin injection process. If the complexcavity is designed to receive a bushing or an insert, the bushing orinsert can be incorporated into the preform and injected in place toeliminate some higher level assembly and to avoid the need for a complextooling surface. If the part includes an internal hollow tube, properdesign of the tool to take this into account may be difficult andexpensive, or may produce a tooling configuration from which removal ofthe finished part would be difficult.

[0014] Other problems are encountered in laying up or arranging preformsof fibers prior to placing the preform into the mold. If braided orwoven fabric is used, cutting of that fabric often results in frayededges, which is undesirable. Arranging stacks, or tapered-off sectionsof the preforms on a substrate so that ply drops are aligned correctlyis also difficult.

[0015] The present invention solves many of the above problems byproviding a series of unique processes for the fabrication of a windtunnel blade. The processes result in a new structure for a wind tunnelblade.

[0016] It has become conventional practice in the aircraft industry tomanufacture helicopter and other blades having a molded fiber-reinforcedresin body formed by resin transfer molding. The fiber-reinforced resinbodies were often formed about an internal, metallic, load-bearing spar.Such fiber-reinforced resin bodies exhibited high strength and lowweight characteristics. With the exception of the internal metal spar,however, prior art resin transfer molded rotor blades did not includestructural reinforcements along their length.

[0017] Prior art wind tunnel blades were formed from a lay-up of prepregcomposite material that was shaped into a unitary structure including abase attached to the blade. The housing and the hub for the wind tunnelblades required that a technician lay on his back and install theunitary base and blade structure into the wind tunnel's hub, which wasdifficult.

[0018] Because prior art wind tunnel blades were subjected to high speedwind conditions, the wind tunnel blades were often damaged as a resultof fatigue and wind erosion. To counter this wind erosion, the prior artwind tunnel blades included frangible foam tips at their distal ends.The frangible foam tips were often formed of a foam material having auniform density. The frangible foam tip was wrapped in plies offiberglass to protect the foam from wind erosion and to improve impactresistance. This wrapped fiber piece was difficult to form, and requireda large amount of labor to produce.

[0019] Prior art wind tunnel blades were difficult to balance becausethe wind tunnel blades were not of uniform weight and did not haveconsistent centers of gravity. The prior art wind tunnel blades werebalanced by adding lead weights to the blade butt to adjust the centerof gravity. After the center of gravity was adjusted, the blade must bematched to another blade of approximately the same weight. This matchingprocess can be difficult because of the large blade-to-blade variationin weight.

[0020] The present invention solves the above problems by providing anovel wind tunnel blade design incorporating a variety of differentfeatures that permit easier installation, service, and replacement ofthe wind tunnel blades. The process of forming the unique wind tunnelblade incorporates a number of new composites forming techniques. Thesetechniques are applicable to a number of parts or products, and can beused to form parts having a number of different configurations orcomplex shapes.

SUMMARY OF THE INVENTION

[0021] The present invention provides a plug including a flexible outerbushing having first and second ends, a connector attached to the firstend of the bushing, and a fastener extending along the flexible outerbushing and attached to the connector. The fastener is configured suchthat actuation of the fastener causes the flexible outer bushing toexpand outward, whereby the flexible outer bushing can be inserted intoa hollow opening and can expand against the sides of the opening byactuation of the fastener.

[0022] In one embodiment, the connector is a female-threaded insert. Thefastener can extend along the bushing and includes (1) an abutmentsurface for engaging the second end of the bushing and (2) male threadsthat are received in the female-threaded insert. Actuation of thefastener involves rotating the fastener to move the connector towardsthe second end.

[0023] In accordance with another aspect of the plug, the fastenerextends along the bushing and comprises an abutment surface for engagingthe second end of the bushing and actuation of the fastener comprisescausing the fastener to pull the connector toward the abutment surface.

[0024] The present invention also provides a method of resin transfermolding a product having a hollow tube therein. The method includesplacing an expandable plug into a hollow tube so that a portion of theplug extends along the intended finished line of the product beingformed, and expanding the expandable plug so that the expandable plug ispressed against the outer sides of the hollow tube. Resin is injectedabout the hollow tube and around the plug in a resin transfer moldingprocess such that excess resin is formed beyond the intended finishline. The excess resin and the expandable plug are cut along theintended finish line so that the plug is no longer expanded and fallsout of the hollow tube.

[0025] The present invention further provides a reinforced corestructure for use in a resin transfer molding process. The reinforcedcore structure includes an expanded core having a longitudinal axis, afirst set of braided fibers extending from a first end of the expandedcore to a first location and reversing from the first groove over itselfand back towards the first end, and a second set of braided fibersextending from the first end over the first set of braided fibers and toa second location beyond the first location and reversing from thesecond location, back over itself and rearward to the first end.

[0026] In one embodiment, the expanded core includes a plurality ofgrooves extending transverse to the longitudinal axis.

[0027] In accordance with another aspect of the invention, a firstgroove is located at the first location, and a first cord ties off thefirst set of braided fibers and extends between the overlapped layers ofthe first set of braided fibers and opposite the first groove so thatthe first cord presses the first set of braided fibers into the firstgroove. A second groove can be provided that is located at the secondlocation. A second cord ties off the second set of braided fibers andextending between the overlapped layers of the second set of braidedfibers and opposite the second groove so that the second cord pressesthe second set of braided fibers into the second groove.

[0028] Preferably, the perimeter of the expanded core between the firstand second grooves is substantially the same as the perimeter of theexpanded core in the region between the first groove and the end and theoverlapped layers of the first set of braided fibers extending over thislatter area.

[0029] A third set of braided fibers can be provided that extends fromthe first end, past the first and second grooves, to a third groovebeyond the second groove and reversing at the third groove over itselfand back to the first end. A third cord can be provided that ties offthe third set of braided fibers and extends between the overlappedlayers of the third set of braided fibers and opposite the third grooveso that the third cord presses the third set of braided fibers into thethird groove.

[0030] Preferably, the perimeter of the expanded core between the firstand second grooves and the overlapped layers of the second set ofbraided fibers extending thereover is substantially the same as theperimeter of the expanded core in the region between the second andthird grooves.

[0031] The present invention further provides a method of forming areinforced core structure for use in a resin transfer molding process.The method includes providing an expanded core having a longitudinalaxis, braiding a first set of fibers from a first end of the expandedcore to a first location on the expanded core, and reversing thedirection of the braiding of the first set of fibers at the firstlocation and continuing braiding back to the first end so that the firstset of braided fibers is braided back upon itself to form a first duallayer fiber structure. A second set of fibers is braided over the firstset of braided fibers from the first end beyond the first location to asecond location. The braiding direction of the second set of fibers isreversed at the second location back toward the first end so that thesecond set of braided fibers is braided back upon itself to form asecond dual layer fiber structure.

[0032] In accordance with one aspect of the method, the first set ofbraided fibers are tied at the first location with a cord beforereversing direction of the braided fibers. The second set of braidedfibers are tied at the second location with a cord before reversingdirection of the braided fibers.

[0033] The expanded core can be provided with a plurality of groovesextending transverse to the longitudinal axis. A first groove is locatedat the first location, and the first set of braided fibers is tied witha cord before reversing direction of the first set of braided fibers.The cord is arranged opposite the first groove such as to pull the firstset of braided fibers into the first groove. A second groove is locatedat the second location, and the second set of braided fibers is tiedwith a cord before reversing direction of the second set of braidedfibers. The cord is arranged opposite the groove such as to pull thesecond set of braided fibers into the second groove.

[0034] The method further provides braiding a third set of fibers fromthe first end over the first and second sets of braided fibers to beyondthe second groove to a third groove and reversing the braiding directionof the third set of fibers at the third groove back toward the first endso that the third set of braided fibers is braided back upon itself toform a third dual layer fiber structure.

[0035] In accordance with another aspect of the present invention, amethod of preparing a reinforced core structure for a product to beformed in a resin transfer molding process utilizing a resin isprovided. The method includes applying fibers over a core beyond thefinal finished line for the product to be formed, applying a tackifiersolution to the fibers located at the final finish line, the tackifiersolution comprising a reduced resin concentration from the final resinconcentration of the product to be formed in the resin transfer moldingprocess, locally consolidating the tackifier solution, and cutting alongthe final finish line.

[0036] Preferably, the tackifier solution includes resin to be used forthe resin transfer molding process diluted by a solvent.

[0037] The present invention further provides a radius filler for use ina resin transfer molding system. The radius filler includesunidirectional tows and a braided sleeve of fibers extending around theunidirectional tows. A tackifier solution can be added to the braidedsleeve, the tackifier solution comprising a diluted mixture of the resinto be used in the resin transfer molding system. The tackifier solutioncan include resin to be used for the resin transfer molding processdiluted by a solvent.

[0038] The present invention further provides a method of forming aradius filler for use in forming a preform to be used in a resintransfer molding process, the method including providing unidirectionaltows, and braiding a sleeve of fibers around the unidirectional tows. Atackifier can be applied to the braided sleeve, the tackifier includinga diluted solution including the resin to be used in the final resintransfer molding process. The tackifier is consolidated so as to lendrigidity to the radius filler.

[0039] The present invention further provides a method of forming a corestructure including providing a mold having an internal cavity,arranging a prepreg along the inside of the internal cavity, the prepregbeing of a size such that the prepreg can extend around a circumferenceof the mold, placing an expandable foam material in the cavity of themold and within the prepreg material, heating the expandable foammaterial so as to expand the foam material within the prepreg materialso to press the prepreg material against the walls of the cavity of themold, and curing the expandable foam material and the prepreg materialso as to form the core structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0041]FIG. 1 shows a side perspective view of a wind tunnel blade madein accordance with the process of the present invention, the wind tunnelblade shown as mounted on a base that attaches to a wind tunnel fan hub;

[0042]FIG. 2 is an exploded side perspective view of the wind tunnelblade and base of FIG. 1;

[0043]FIG. 3 is a sectional view taken along the section lines 3-3 ofFIG. 1;

[0044]FIG. 4 is a side perspective view of adjacent cuffs for adjacentwind tunnel blades such as is shown in FIG. 1;

[0045]FIG. 5 is a sectional view taken along the section lines 5-5 ofFIG. 4;

[0046]FIG. 6 is a top view of three foam core sections used to make thewind tunnel blade of FIG. 1;

[0047]FIG. 7 is a perspective view of the wind tunnel blade of FIG. 1,with the core sections of FIG. 6 shown in phantom;

[0048]FIG. 8 shows a top plan view of the bottom mold for making thecentral core section of FIG. 6;

[0049]FIG. 9 shows the core for the central core section of FIG. 6;

[0050]FIG. 10 shows a diagrammatic cutaway view of an expandable plugfor use in formation of the core of FIG. 9;

[0051]FIG. 11 shows the expandable plug of FIG. 10 in an expandedposition and positioned within a metal tube;

[0052]FIG. 12 shows a diagrammatic side view of the core of FIG. 9, withbraided fibers being applied around one end;

[0053]FIG. 13 is a diagrammatic side view similar to FIG. 12, with aportion of the braided fibers being tied off within a groove on thecore;

[0054]FIG. 14 is a diagrammatic side view similar to FIGS. 12 and 13,showing the braided fibers being braided onto the core in an oppositedirection over the first layer of braided fibers;

[0055]FIG. 15 is a diagrammatic side view similar to FIG. 14, showingadditional braided fibers extending over the first braided fibers;

[0056]FIG. 16 is a diagrammatic side view similar to FIG. 15, with thesecond braided fibers in position;

[0057]FIG. 17 is a diagrammatic side view similar to FIG. 16, with fivebraided fibers in place on the outside of the core;

[0058]FIG. 18 is a diagrammatic side view similar to FIG. 7, withadditional braided fibers over the outside of the core;

[0059]FIG. 19 is a side perspective view of the finished braided centralcore section shown in FIGS. 9-18;

[0060]FIG. 20 shows the application of a tackifier to the end of thecentral core section of FIG. 19;

[0061]FIG. 21 shows shrink tape being applied over the tackifier that isapplied in FIG. 20;

[0062]FIG. 22 shows the central core section of FIGS. 19-21 placed in aframe prior to cutting;

[0063]FIG. 23 is a sectional view of the central core section of FIG.22, taken along the sectional lines 23-23;

[0064]FIG. 24 is a sectional view of the wind tunnel blade of FIG. 7,taken along the sectional lines 24-24;

[0065]FIG. 25 is a detailed view of a radius filler formed in accordancewith the present invention, taken in the detail section 25 of FIG. 24;

[0066]FIG. 26 shows a mandrel for formation of the radius filler of FIG.25;

[0067]FIG. 27 is a sectional view of the mandrel of FIG. 26, taken alongthe sectional lines 27-27 and showing a vacuum bag in place over themandrel;

[0068]FIG. 28 shows the cut core sections of FIG. 6 in place and beingwrapped by a prepreg sheet;

[0069]FIG. 29 shows a mold in which the fan tunnel blade of FIG. 6 isformed;

[0070]FIG. 30 is a diagrammatic view of the wind tunnel blade of FIG. 6,as removed from the mold of FIG. 29, and displaying a cut line alongwhich the wind tunnel blade is cut before finishing;

[0071]FIG. 31 shows a mold in which the tip for the wind tunnel blade ofFIG. 1 is formed, the mold having foam material and an outer skintherein;

[0072]FIG. 32 is a diagrammatic view of the mold of FIG. 31, with thefoam material expanded and the outer skins pressed against the innermold line of the mold;

[0073]FIG. 33 is a top view of the wind tunnel blade tip formed in themold of FIGS. 31 and 32; and

[0074]FIG. 34 is a side perspective view of a balance mechanism that isfitted within the wind tunnel blade of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0075] Referring now to the drawing, in which like reference numeralsrepresent like parts throughout the several views, FIG. 1 shows a windtunnel blade 30 made in accordance with the present invention. The windtunnel blade 30 is connected to a base 32 and is held in position by atwo-piece cuff 34.

[0076] Briefly described, the present invention is directed to theseparate wind tunnel blade 30, base 32 and cuff 34 system, and theunique configuration and structure of the wind tunnel blade 30. Inaddition, the present invention is directed to resin molding transferprocesses for forming the wind tunnel blade 30.

[0077] The base 32 is designed to be attached to a hub of a wind tunnelfan (not shown). A plurality of the wind tunnel blades 30 projectradially outward from the hub and are supported therefrom in aconventional fashion at the base 32. Any number of wind tunnel blades 30can be used with the wind tunnel fan. As a nonlimiting example, the windtunnel blade 30 shown in FIG. 1 is for use with a wind tunnel fan havingseventy-two (72) wind tunnel blades spaced circumferentially about thehub for the wind tunnel fan. Since each wind tunnel blade 30 isidentical, only a single wind tunnel blade will be described in thisdisclosure.

[0078] The base 32 is preferably cast aluminum, and includes a pedestal36 (best shown in FIG. 2) that is configured to be attached to arotating wind tunnel fan hub (not shown, but well known in the art). Thepedestal 36 includes two legs 38 and five elongate bores 40 extendingtherethrough. The elongate bores 40 receive bolts (not shown) that areattached to the rotary wind tunnel fan hub in a manner known in the art.

[0079] A series of flanges 42 extend orthogonally from top corners ofthe sidewalls of the pedestal 36. The flanges 42 include downwardlyextending mounting holes 44 that are configured to receive bolts (notshown) that extend through the cuff 34.

[0080] The cuff 34 includes a fore cuff piece 45 and an aft cuff piece46 (best shown in FIG. 2). The fore and aft cuff pieces 45, 46 includeholes 48 through which mounting bolts extend into the mounting holes 44on the base 32. The fore cuff piece 45 and the aft cuff piece 46 includeangled cuts 50, 52 (FIG. 3) that are configured to extend along andunderneath the bottom edges of the wind tunnel blade 30. The angled cuts50, 52 are preferably cut, one at an acute angle, the other at an obtuseangle, to the top plane of the fore cuff piece 45 and the aft cuff piece46, and are arranged so that they fit together to form a smoothtransition between the fore cuff piece 45 and the aft cuff piece 46. Theangled cuts 50, 52 are shaped so that the surfaces of the two cutsextend forwardly and downwardly from the top surface of the fore and aftcuff pieces 45, 46. In this manner, air flow (see FIG. 3) over the topsurface of the cuff pieces 45, 46 is not directed into the joint betweenthe cuff pieces.

[0081] The fore cuff piece 45 and the aft cuff piece 46 are preferablycompression molded using fiberglass in an epoxy resin. As can be seen inFIG. 5, the forward portion of the fore cuff piece 45 is formed with astepped-downward section 54. A trailing, flat section 56 of the aft cuffpiece 46 fits over the stepped-downward section 54 of the fore cuffpiece 45. The stepped-down section 54 and the flat section 56 form aninterconnected lap joint that is attached by nutplates (not shown, butof a typical model designed for composites) so that a series of thecuffs 34 creates a passive p-seal between the cuffs 34 (FIG. 4) thatextend over the wind tunnel fan hub (not shown).

[0082] In summary, the cuff 34 includes three design features thatcontribute to preventing airflow leakage downward through the cuffs intothe wind tunnel fan hub. First, the angled cuts 50, 52 form a split linethat is angled away from air flow over the cuff 34 and is covered by thebottom edge of the wind tunnel blade 30. Second, the cuffs 34 are linkedtogether so as to form a passive p-seal over the wind tunnel fan hub.Finally, the interconnected lap joint between the stepped-down section54 of the fore cuff piece 45 and the flat section 56 of the aft cuffpiece 46 prevents further leakage.

[0083] Referring back to FIG. 1, the wind tunnel blade 30 includes arearward edge 62 and a rounded, leading edge 64. The wind tunnel blade30 includes a tang root 66 (FIG. 2) that extends into a slot within thebase 32 and attaches to the base 32 by bolts (not shown) through holes65 in the tang root. The tang root 66 includes a protrusion 67 at theintersection of the tang root and the wind tunnel blade 30 that extendsperpendicular to the longitudinal axis of the wind tunnel blade andalong the aft, right top portion of the tang root 66. The aft cuff piece46 fits over the protrusion 67, to add further stability and to help fixthe wind tunnel blade 30 in position.

[0084] The wind tunnel blade 30 includes a separate tip 68 attachedalong the distal end of the wind tunnel blade. The function and thestructure of the tip 68 are described in detail below.

[0085] The method of forming the wind tunnel blade 30 will now bedescribed. Describing the process broadly with reference to FIG. 6, thewind tunnel blade 30 is formed from a central foam core section 70, afore foam core section 72, and an aft foam core section 74, each ofwhich extends longitudinally the length of the blade. The central, fore,and aft core sections 70, 72, 74 are attached along their surfaces toform the body of the blade (FIG. 7). The central foam core section 70extends downward beyond the bottom ends of the fore and aft coresections 72, 74 to form the tang root 66. The tip 68 is attached alongthe distal end of the foam core sections 70, 72, 74.

[0086] A bottom mold 76 for forming a foam core 83 (FIG. 9) for thecentral foam core section 70 is shown in FIG. 8. The cavity 77 for thebottom mold 76 substantially matches the outer contour of the desiredfinished product; i.e., the foam core 83. The mold cavity 77 includesprotrusions 78 a-e that extend transverse to the longitudinal axis ofthe bottom mold 76, and extend around the perimeter of the cavity,including a top for the mold (not shown) that is closed over the bottommold 76 to form a closed cavity for formation of the foam core 83. Ametal tube 80 is suspended by a wire 81 within the cavity 77 of the mold76. Preferably, the wire 81 extends upward from the bottom of the cavity77 (not shown). In the embodiment shown, the wire 81 is welded to theend of the metal tube 80, and extends into a small hole 82 at the end ofthe mold cavity 77. Many other arrangements for suspending the metaltube 80 with a wire can be used.

[0087] As is described in detail below, the foam core 83 is formedaround the metal tube 80 so that the metal tube and the wire 81 become apart of the foam core and later a part of the central foam core section70. The top portion of the metal tube 80 is plugged so as to prevent theflow of core therein. The bottom of the metal tube fits against aprotrusion (not shown) in the mold cavity 77 to prevent flow in thatend. The metal tube 80 is arranged so as to extend from the bottomportion of the foam core 83 (i.e., the end that forms the tang root 66)to a location approximately two-thirds of the length up the central foamcore section 70. The metal tube 80 is designed to receive a balancemechanism 82 (FIG. 34), the function and structure of which aredescribed in detail below. The metal tube 80 is preferably cylindricallyshaped and formed from aluminum, but any appropriately shaped metal orother suitable material can be used.

[0088] After the metal tube 80 is in place, a polyurethane foam mixture(not shown) is poured into the mold cavity 77 and encapsulates the metaltube 80. The top mold (not shown) is placed over the bottom mold 76 toseal the cavity 77. The polyurethane foam mixture is heated untilexpanded to fill the mold and is held at a curing temperature untilhardened. The polyurethane foam mixture and the metal tube 80 thus forma unitary structure of the foam core 83 for use in forming the centralfoam core section 70. The length of the foam core 83 after it is removedfrom the mold 76 is slightly longer than the final central foam coresection 70 used to form the wind tunnel blade 30. The excess lengthrepresents excess foam at each end of the foam core 83 that is removedafter a braided fiber shell has been placed around the foam core 83, asis described in detail below.

[0089] As can be seen in FIG. 9, the final foam core 83 includesindentations, or grooves 85 a-e that extend around the circumference ofthe foam core 83. The grooves 85 a-e are formed by the protrusions 78a-e in the mold cavity 77. The foam core 83 also tapers in circumference(i.e., decreases in perimeter) as the foam core approaches the bottomend (i.e., the tang root end). The decreases in perimeter occur insteps, and each of the steps begins at one of the grooves 85 a-e. Thefunctions of the stepped decreases in perimeter and the grooves aredescribed in detail below.

[0090] An expandable plug 87 (FIG. 10) is placed in the open end of themetal tube 80. The expandable plug 87 includes a threaded fastener 88that extends through a rubber-faced metal washer 89 and into a rubberbushing 90. The rubber bushing 90 has an internal diameter thatsubstantially matches the outside diameter of the threaded fastener 88.The outer diameter of the bushing 90 is slightly smaller than the innerdiameter of the metal tube 80. A flange 92 extends around thecircumference of the top end of the rubber bushing 90. A threaded insert94 is located within the internal circumference of the bore for therubber bushing 90. A tool-receiving pattern 95 is located at the top endof the threaded fastener 88.

[0091] The expandable plug 87 is placed in the end of the metal tube sothat the flange 92 fits over the outer circumference of the metal tube.A tool, such as a screwdriver, is placed in the tool-receiving pattern95 of the threaded fastener 88. The threaded fastener 88 is then rotatedinto the threaded insert 94 until the rubber face metal washer 89 ispressed against the flange 92 on the rubber bushing 90. Continuedrotation of the threaded fastener 88 causes the rubber bushing to buckle(FIG. 11), and press outward on the sides of the metal tube 80, thussealing the end of the metal tube 80.

[0092] After the expandable plug 87 is in place, fibers 99 (FIG. 12) arebraided around the foam core 83, beginning at the bottom end so as toform a fiber sock 100 a. The braided fiber sock 100 a is preferablyformed from fiberglass fibers, but can be graphite, aramid, ceramics, orany other suitable material. The fibers 99 are preferably braided ontothe foam core 83, but can be knitted, woven, filament-wound, or stitchedonto the foam core. Braiding results in the fibers being in an orientedpattern around the entire circumference of the foam core 83. The braidedfibers 99 also form a snug-fitting preform around the foam core 83.

[0093] The fibers 99 are continually braided up the circumference of thefoam core 83 until the braided fiber sock 100 a extends beyond the firstgroove 85 a (FIG. 12). A cord 102 a (FIG. 13) is then placed around thebraided fiber sock 100 a opposite the first groove 85 a. The cord 102 ais preferably made of fiberglass; but any other suitable material can beused. The cord 102 a is tensioned and tied off such that the braidedfiber sock 100 a extends downward into the groove 85 a (FIG. 13). Thebraiding direction of the fibers is then reversed such that the braidedfiber sock 100 a overlaps itself (FIG. 14) and extends back to andbeyond the bottom end of the foam core 83. The braided fiber sock 100 ais then cut, and the free ends are permitted to dangle beyond the end ofthe foam core 83.

[0094] Preferably, the groove 85 a is of a depth and size so that thefold in the braided fiber sock 100 a where the braided fiber sockreverses direction is contained within the groove 85 a, and thus asmooth surface is maintained at the transition (FIG. 14). Moreover, theperimeter of the foam core 83 between the groove 85 a and the end of thefoam core is a sufficient amount less than the perimeter between thegrooves 85 a and 85 b such that, once the braided fiber sock 100 a hasbeen put in place, the outer surface of the return layer of the braidedfiber sock is level with the outer circumference of the foam core 83between the grooves 85 a and 85 b (FIG. 14).

[0095] After the braided fiber sock 100 a is extended beyond the bottomof the foam core 83, the fibers are cut and a second braiding processbegins from the bottom of the core over the initial braided fiber sock100 a. Instead of cutting the initial braided fiber sock 100 a, thedirection of braiding of the fibers can be reversed, and the A secondbraided fiber sock 100 b (FIG. 15) is formed over the initial braidedfiber sock 100 a and over the outer circumference of the foam core 83between the grooves 85 a and 85 b. Instead of cutting the initialbraided fiber sock 100 a, the direction of braiding of the fibers can bereversed, and the second braided fiber sock 100 b can be formed bycontinued braiding of the first fiber sock. This method is preferred tocutting, because it does not produce frayed edges that must be kept inorder.

[0096] The second braided fiber sock 100 b is extended beyond the groove85 b (FIG. 15) and a second cord 102 b is tensioned and tied over thesecond braided fiber sock 100 b and pulled downward into the groove 85b. The direction of the braid for the braided fiber sock 100 b is thenreversed, and the braided fiber sock 100 b extends rearward beyond thebottom of the foam core 83 (FIG. 16). The second fiber sock 100 b isthen cut, and a third fiber sock 100 c is formed over the second fibersock 100 b (alternatively braiding is reversed, as described above).This process is continued until all of the grooves 85 a-e have beenfilled, and five braided fiber socks 100 a-e extend to the respectivegrooves 85 a-e (FIG. 17), and extend rearward beyond the bottom end ofthe foam core 83. A final braided fiber sock 103 (FIG. 16) is thenformed along the length of the foam core 83 over the braided fiber socks100 a-e and the exposed portion of the foam core 83. The final braidedfiber sock 103 extends beyond both ends of the foam core 83.

[0097] The fore and aft foam core sections 72, 74 include foam cores104, 105 (FIG. 24) formed in a manner similar to the foam core 83. Thatis, the foam cores 104, 105 are covered by braided fiber socks 106, 107.The braided fiber socks 106, 107 are placed on the foam cores 104, 105so that the braided fiber socks extend beyond both ends of the foamcores. Unlike the foam core 83 for the central foam core section 70, thefoam cores 104, 105 do not include step sections. Instead, only a singlelayer of fibers (the braided fiber socks 106, 107) extend the entirelength of the fore and aft foam core sections 72, 74. Any number oflayers of the braided fiber socks 106, 107 may be used over the foamcores 104, 105, but in the preferred embodiment, only one layer of thebraided fiber sock is used on each of the foam cores.

[0098] After the central, fore, and aft foam core sections 70, 72, 74are formed, the ends of the foam core sections are cut so as to removeexcess material from the ends of the foam cores 83, 104, 105 and theexcess braided fiber socks 100 a-e, 103, 106, and 107. To cleanly cutthe braided fiber socks 100 a-e, 103, 106, and 107, a unique process hasbeen developed. Because each of the foam core sections 70, 72, 74 arepreferably cut in the same manner, the cutting process for only thecentral foam core section 70 will be described.

[0099] A tackifier 112 is applied by a brush 109 (FIG. 20) to the endsof the central foam core section 70. The tackifier 112 is preferably thebase resin that will be used in the final resin transfer molding processof the wind tunnel blade 30, diluted in a solvent such as acetone. Thetackifier 112 is applied at the location of the cuts, and overlaps thecuts in both directions by approximately half an inch. The tackifier 112is applied in sufficient quantities to saturate through each of thebraided fiber socks 100 a-e, 103.

[0100] The tackifier 112 is locally consolidated by such methods asvacuum bag, shrink tape, or hard tooling until the polymer material isstable due to cooling of the hot melt or by flashing of the solvent fromthe solution. In the embodiment shown, shrink tape 113 (FIG. 21) isapplied around and over the portion of the braided fiber socks 100 a-e,103 that has been saturated with the tackifier 112. The shrink tape 113is heated to apply pressure and heat to the tackifier 112, causing theshrink tape 113 to constrict around the central foam core section 70 andapply pressure until the tackifier 112 precures (i.e., semi-hardens).

[0101] The central foam core section 70 is then removed from the ovenand placed in a frame 108 (FIG. 22). The frame 108 is designed as afour-sided box having sides 109 and ends 110. The sides 109 extendbeyond the side edge of the central core section. The ends 110 of theframe 108 are spaced apart a length that is the same as the length ofthe finished central foam core section 70. The top surface of the ends110 include indentations 111 (FIG. 23) that are designed to receive andsupport the ends of the central foam core section 70.

[0102] The uncut central foam core section 70 is placed on the frame 108such that the excess materials for the foam core 83 and braided fibersocks 110 a-e and 103 extend beyond the ends 110 of the frame. Theportions of the central foam core section 70 upon which the tackifier112 was applied align with the ends 110 of the frame.

[0103] After the central foam core section 70 is placed on the frame108, brackets 114 are placed over opposite ends of the central foam coresection opposite the ends 110 of the frame. The brackets 114 includeindentations 115 (FIG. 23) that substantially match the contour of theupper side of the central foam core section 70. Thus, the brackets 114and the ends 110 of the frame 108 work together to encase the centralfoam core section 70 at opposite ends of the central foam core section.The brackets 114 and the ends are then attached so as to hold thecentral foam core section 70.

[0104] The central foam core section 70 is then cut just along the outeredges of the ends 110 and brackets 114. The fact that the central foamcore section 70 is clamped between the indentations 111, 115 on the ends110 and the brackets 114 ensures that the central foam core section 70is stable during the cutting process. In this manner, the braided fibersocks 100 a-e, 103 are not pulled away from the foam core 83, and damageto the foam core 83 during the cutting process is minimized.

[0105] The tackifier 112 seals the braided fiber socks 100 a-e, 103against the outer surface of the foam core 83 and prevent fraying of thefiberglass within the braided fiber socks upon cutting of the socks. Inthis manner, smooth cuts are formed at the ends of the central foam coresection 70.

[0106] The fore and aft foam core sections 72, 74 are prepared and cutin the same manner as the central foam core section 70. The central,fore, and aft foam core sections 70, 72, 74 are now ready for assembly.

[0107] As stated above, the central, fore, and aft foam core sections70, 72, 74 are placed together to form the wind tunnel blade 30. Becausethe edges of the central, fore, and aft foam core sections 70, 72, 74are rounded, radius fillers 120 (FIG. 24) are used to fill the gapsbetween the outer edge of the foam core sections. The radius fillers 120used in the wind tunnel blade 30 are formed using a novel process. Inaccordance with the process, a braided sleeve 122 surrounds a number ofunidirectional tows 124 (FIG. 25). The unidirectional tows 124 can beinserted into the bi-axial braided sleeve 122, or the braided sleeve canbe formed around the unidirectional tows.

[0108] The core of the unidirectional tows 124 can be of uniform crosssection, or can be varied in cross-section along its length so as to fita particular gap. The radius fillers 120 of the wind tunnel blade 30have a substantially uniform triangular cross-section, with tworadiused, or curved sides 125. The curved sides 125 correspond to thesides that abut against adjacent foam core sections.

[0109] The radius filler 120 is formed on a mandrel 127 (FIG. 26) thatincludes a contoured surface that is substantially the same as thejuncture of the two foam core sections 70, 72 or 70, 74, between whichthe radius filler will be placed. In the present invention, the mandrel127 includes a first radiused mandrel surface 126 adjacent to a secondradiused mandrel surface 128. The first radiused mandrel surface 126 inthe example shown in the drawings is a pipe that has a radius that issubstantially the same as the outer radius of the fore foam core section72. The second radiused mandrel surface 128 is a machined metal that hasa radius that is substantially the same as the outer radius of thecentral foam core section 70.

[0110] The braided sleeve 122 is braided around the unidirectional tows124, and is then soaked with a tackifier that is similar in compositionto the tackifier 112 described in detail above. The braided sleeve 122with the unidirectional tows 124 therein is then placed between the tworadiused mandrel surfaces 126, 128, and is vacuum bagged under a bladder130 (FIG. 27). The bagged radius filler 120 is then placed in anautoclave (not shown) and heat is applied while vacuum is applied to thebladder 130. The bagged radius filler 120 is heated until the tackifieron the braided sleeve 122 is precured, or semi-hardened.

[0111] The tackifier solution that is placed on the braided sleeve 122places a resin coating over the braided sleeve so that the resin equalsapproximately 6% of the weight of the fibers in the resin. In contrast,in the final resin transfer molding process, the resin is approximately50% of the weight of the resin and fiber composite. The amount of resinin the tackifier is preferably sufficient to maintain or hold the shapeof the radius filler 120 after precuring, but is not sufficient toharden it into a rigid, cured state. Thus, the tackifier works as abinding agent to maintain consolidation and configuration of the braidedsleeve 122 until the final resin transfer molding of the wind tunnelblade 30.

[0112] Each of the radius fillers 120 for the wind tunnel blade 30 areformed in a manner similar to the process described above. However, theradiused mandrel surfaces 126, 128 may have a different contour so as toproduce radius fillers that fit between the respective foam coresections 70, 72, and 74.

[0113] The formed central, fore, and aft foam core sections 70, 72, 74and the radius fillers 120 are then consolidated into the shape of thewind tunnel blade 30. The entire assembly is laid over a tackified sheet131 (FIG. 28) on a lay-up mandrel (not shown). The tackified sheet 131is wrapped over the top of the assembly and is trimmed to fit theassembly. The assembly and the tackified sheet 131 are then vacuumbagged and precured. The consolidated assembly, called a “preform,” isthen ready for resin transfer molding.

[0114] The preform is removed from the lay-up mandrel and is placedwithin a bottom mold 133 (FIG. 29) for the resin transfer moldingprocess. The bottom mold 133 is contoured to the tang 60, leading edge62, rearward edge 64, tang root 66, and the protrusion 67 of the windtunnel blade 30. The bottom mold 133 includes an inlet 134 adjacent tothe tang root 66, but is spaced approximately two inches therefrom. Anoutlet 135 is located at the top end of the bottom mold 133 for theoutflow of resin. The bottom mold 133 includes inner and outer O-rings136, 137, which provide a primary and secondary seal between the bottommold and a top mold (not shown). The two O-rings improve sealperformance to maintain vacuum during the resin transfer moldingprocess, and the second seal 137 provides a backup to the primary seal136 in case of primary seal failure.

[0115] The preform is carefully positioned in the bottom mold 133 withindex locators. Once the preform has been set in place, the partingplanes are inspected for possible ply mislocation or obstruction thatwill cause ply pinch and mold closure interference.

[0116] After the tool has been closed and the plumbing attached, thesystem is checked for vacuum integrity. This is commonly done with thevacuum source and a vacuum gauge at the resin trap. Shutoff valves canisolate the plumping for the entire system. After applying high vacuum,the system is allowed to stand static for up to five minutes to verifythe level of vacuum stability. The vacuum assists the resin flow throughthe complex shapes with minimal porosity.

[0117] The bottom mold 133 and the upper mold are then heated to theresin system injection temperature, and the resin system is injectedinto the mold through the inlet port 134. The expandable plug 87 in themetal tube 80 prevents the resin system from flowing into the metaltube. The resin fills the void at the bottom end of the bottom mold 133between the inlet 134 and the tang root 66. In addition, the resinpenetrates all of the preforms within the system, including the braidedfiber socks 100 a-e, 103, 106, 107 and the tackified sheet 131. Theinlet 134 and the outlet 135 are used to deliver the resin to and fromthe mold.

[0118] The resin for the wind tunnel blade 30 is preferably Epon dpl 862RTM liquid resin with the Epon curing agent W added as a curingadditive, available from Shell Chemical Company, but other resins orother resin systems can be used. When selecting a resin for a transfermolding resin design, the first step is to clearly define theperformance conditions. Some of the performance criteria include therange of operating temperatures, thermal cycles, and mechanicalproperties. To insure the proper resin selection, the resin propertiesmust be evaluated based on the performance conditions. A wide variety ofresin systems are available for use in the present invention, along withmany others that are in the development stage. Some of the generic resintransfer molding resin systems that can be used include: epoxy resinsystems; cyanate ester resin systems; vinyl ester resin systems;phenolic resin systems; polyester resin systems; and bismaleimide resinsystems.

[0119] Ideally, the resin injection procedure creates a constant-flowfront, with complete fiber wet-out on a microscopic level, and achievestotal mold cavity fill. The recommended way to create a constant-flowfront is to use an injection system that maintains positive displacementat low pressure. Sustaining a low resin viscosity through the injectioncycle helps to control the pumping pressure. Another aid to achievingtotal fiber wet-out and mold fill is to conclude the injection cyclewith an appropriate hydrostatic pressure. The hydrostatic pressureshould be maintained until the resin matrix is well within its gelphase. The level of hydrostatic pressure is governed by the type ofresin system, mold design, and supporting equipment.

[0120] After the resin is completely injected into the preform, thetemperature of the mold is increased to the cure temperature for theresin system. The mold is held at this temperature for a sufficient timeto cure the resin. After curing is complete, the wind tunnel blade 30 isremoved from the mold and the excess resin 66 a at the tang root 66 issheared off along the line 66 b shown in FIG. 30. When the excess resin66 a is cut off the tang root 66, the metal tube 80 and the expandableplug 87 are also cut, generally along the dotted line 66 b shown in FIG.30. After the excess resin 66 a and the portions of the metal tube 80and expandable plug 87 are removed, the threaded fastener 88 is cut inhalf, releasing the rubber bushing 90 of the expandable plug 87 so thatthe rubber bushing 90 is no longer forced against the sides of the metaltube 80, and simply falls out. Alternatively and preferably, a cut canbe made so that the entire expandable plug 87 is cut out and falls out,and the tube and the expandable plug 87 (still filly expanded) areseparated from the final product.

[0121] The tip 68 is formed separately from the rest of the wind tunnelblade 30. To form the tip 68, a teardrop-shaped mold 142 having a moldcavity that substantially matches the shape of the tip is used. An outerskin 144, preferably a prepreg sheet of material (fibers impregnatedwith a resin), is placed within the mold 142. The outer skin 144 wrapssubstantially around the inside mold cavity 143 of the mold 142.

[0122] Foam material 150 is placed inside the outer skin 144. The mold142 is then placed in an oven and heated so that the foam material 150expands. During this expansion process, the outer skin 144 is pressedoutward against the mold cavity 143. The resin in the outer skin 144cures during the same process, and a tip 68 is formed (FIG. 33) that hasa foam core with a hard, outer skin 144. The tip 68 is then glued to thetop end of the molded wind tunnel blade 30.

[0123] The balance mechanism 82 is shown in FIG. 34. The balancemechanism 82 is inserted into the metal tube 80 after the resin transfermolding process. The balance mechanism 82 includes a threaded rod 154that extends the length of the metal tube 80. Ballast weights 156 arelocated along the length of the threaded rod 154. A metal plate 158 issecured to the end of the threaded rod 154 by a jam nut 162. An end cap160, that is sized and shaped to fit against the end of the tang root66, is bonded to the end of the tang root. The plate 158 is held againstthe end cap 160 by bolts 164. The bolts 164 extend upward into the endcap 160.

[0124] The distal end of the threaded rod 154 includes a tube cap 166that is sized so as to receive the end of the threaded rod and toposition the threaded rod laterally within the metal tube 80. Each ofthe ballast weights 156 include grooves on the outer surface thereof forreceiving O-rings 168. The O-rings 168 bear against the inner surface ofthe metal tube 80 to minimize vibration of the ballast weights 156.Thus, the O-rings 168 are located along the length of the threaded rod154 and position the threaded rod within the metal tube 80. The ballastweights 156 and O-rings 168 are held between pairs of jam nuts 157.

[0125] In practice, the wind tunnel blade 30 is balanced by the balancemechanism 82. The balance mechanism allows both the weight and thecenter of gravity of the wind tunnel blade 30 to be adjusted. The numberof ballast weights 156 can be varied by removing or adding ballastweights 156 to the threaded rod 154. The position of the ballast weights156 along the threaded rod 154 can be varied by moving the jam nuts 157up and down the length of the threaded rod 154, which in turn moves theballast weights 156 up and down the threaded rod. In this manner, boththe weight and the center of gravity of the wind tunnel blade 30 can beadjusted.

[0126] As can be understood from the foregoing, the present inventionprovides numerous advantages in the structure of the wind tunnel blade30 over wind tunnel blades of the prior art. The separate wind tunnelblade 30, base 32, and cuff 34 provide ease of maintenance. If damage tothe wind tunnel blade 30 occurs, the wind tunnel blade can be releasedfrom the base 32 and the cuff 34, and a new wind tunnel blade can beinstalled. In contrast, in prior art wind tunnel blades, the blade, baseand cuff were a single structure, and had to be replaced upon damage tothe wind tunnel blade.

[0127] The two-piece cuff 34 allows access to the wind tunnel blade 30by removing only one of the fore cuff piece 45 or the aft cuff piece 46.In addition, the cuff 34 minimizes air flow leakage downward through thecuffs into the wind tunnel fan by providing the angled cuts 50, 52 thatform a split line that is angled away from air flow over the cuff 34 andis covered by the bottom edge of the wind tunnel blade 30. In addition,adjacent cuffs 34 are linked together so as to form a passive p-sealover the wind tunnel fan hub. The interconnected lap joint betweenadjacent cuffs 34 is also designed to prevent leakage.

[0128] Resin transfer molding provides smooth finished surfaces on bothsides of the wind tunnel blade 30. In contrast, prior art prepreg lay-upmethods provided a single surface that was formed against a tool andthat was smooth. The smooth surfaces provided by resin transfer moldingprovide an aerodynamic, decorative finish, with controlled fit-upsurfaces.

[0129] The new construction of a tip 68 for the wind tunnel blade 30provides an improved structure and ease of construction not provided bythe prior art.

[0130] The radius filler 120 provides several advantages over prior artradius fillers. In the prior art, radius fillers were most often formedby prepreg materials that were formed into the shape of the radiusfillers. In contrast, the radius filler 120 of the present inventionprovides unidirectional tows 122 within a bi-axial braided sleeve 124.The unidirectional tows 122 can be tailored to accommodate variouscross-sectional areas. In addition, the core of the unidirectional tows122 can be of uniform cross-section or can be tailored to providevarying cross-sectional areas along the length.

[0131] The three-piece core construction of the wind tunnel blade 30provides structural, longitudinal support along the length of the windtunnel blade. Adjacent foam core sections provide I-beams at theirintersections.

[0132] The balance mechanism 82 provides an easy manner in which tomatch the centers of gravity and weight of a large number of wind tunnelblades 30. The balance mechanism 82 is easily adjustable, and is easilyaccessed by removal of the wind tunnel blade 30.

[0133] The stepped braided fibers on the central foam core section 70provide increased strength adjacent to the base 32, and lighter weightnear the tip 38 of the wind tunnel blade 30. The stepped constructiontherefore provides the optimal strength and weight characteristics forthe wind tunnel blade 30.

[0134] The methods of construction of the wind tunnel blade 30 disclosedherein are not only convenient for formation of the wind tunnel blade30, but can also be used for additional parts. For example, theexpandable plug 87 provides an easy manner of plugging a tube within apreform. The expandable plug 87 prevents the flow of resin into themetal tube 80 during the resin transfer molding process, but after beingcut, releases the sides of the metal tube 80 and falls out of the metaltube.

[0135] The tackifier 112 provides a convenient way of stabilizing theedges of reinforced preforms prior to trimming the edges. A tackifier112 is applied to edges to be cut, and is locally consolidated so thatthe fiber preforms are held together during the cutting process. In thisway, the fraying, lofting, and distortion caused by trimming can beavoided.

[0136] The method for providing multiple ply drop off of braided fabricdisclosed herein provides a convenient and efficient manner of providinga reinforced core structure for a composite part. Grooves are providedon the core, and the braided fibers are tied off in the grooves. Thetied off, braided fibers provide a smooth transition on reverse ofdirection of the braiding of the fibers, and permits an additional fiberlayer to be braided over the transition.

[0137] While the preferred embodiment of the invention has beenillustrated and described with reference to preferred embodimentsthereof, it will be appreciated that various changes can be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A plug comprising: (a) aflexible outer bushing having first and second ends; (b) a connectorattached to the first end of the bushing; and (c) a fastener extendingalong the flexible outer bushing and attached to the connector, thefastener being configured such that actuation of the fastener causes theflexible outer bushing to expand outward, whereby the flexible outerbushing can be inserted into a hollow opening and can expand against thesides of the opening by actuation of the fastener.
 2. The plug of claim1, wherein the connector is a female-threaded insert.
 3. The plug ofclaim 2, wherein the fastener extends along the bushing and comprises(1) an abutment surface for engaging the second end of the bushing and(2) male threads that are received in the female-threaded insert, andwherein actuation of the fastener comprises rotating the fastener tomove the connector towards the second end.
 4. The plug of claim 1,wherein the fastener extends along the bushing and comprises an abutmentsurface for engaging the second end of the bushing and actuation of thefastener comprises causing the fastener to pull the connector toward theabutment surface.
 5. A method of resin transfer molding a product havinga hollow tube therein, the method comprising: (a) placing an expandableplug into a hollow tube so that a portion of the plug extends along orbeyond the intended finished line of the product being formed; (b)expanding the expandable plug so that the expandable plug is pressedagainst the outer sides of the hollow tube; (c) injecting resin aboutthe hollow tube and around the plug in a resin transfer molding processsuch that excess resin is formed beyond the intended finish line; and(d) cutting the excess resin along the intended finish line so that theplug is removed from the final finished part.
 6. The method of claim 5,wherein the plug comprises: (a) a flexible outer bushing having firstand second ends; (b) a connector attached to the first end of thebushing; and (c) a fastener extending along the flexible outer bushingand attached to the connector, the fastener being configured such thatactuation of the fastener causes the flexible outer bushing to expandoutward, whereby the flexible outer bushing can be inserted into ahollow opening and can expand by actuation of the fastener against thesides of the opening.
 7. The method of claim 6, wherein the cutting stepcomprises cutting the fastener so that the outer bushing no longerexpands outward and-is free to fall out of the hollow tube.
 8. Themethod of claim 6, wherein the fastener extends along the bushing andcomprises an abutment surface for engaging the second end of the bushingand actuation of the fastener comprises causing the fastener to pull theconnector toward the abutment surface.
 9. The method of claim 5, whereinthe expandable plug is cut during the cutting process so that the plugis no longer expanded against the sides of the hollow tube and falls outof the hollow tube.
 10. A reinforced core structure for use in a resintransfer molding process comprising: (a) an expanded core having alongitudinal axis; (b) a first set of braided fibers extending from afirst end of the expanded core to a first location and reversing fromthe first groove over itself and back towards the first end; and (c) asecond set of braided fibers extending from the first end over the firstset of braided fibers and to a second location beyond the first locationand reversing from the second location, back over itself and rearward tothe first end.
 11. The reinforced core structure of claim 10, whereinthe expanded core comprises a plurality of grooves extending transverseto the longitudinal axis;
 12. The reinforced core structure of claim 11,wherein a first groove is located at the first location, and furthercomprising: a first cord tying off the first set of braided fibers andextending between the overlapped layers of the first set of braidedfibers and opposite the first groove so that the first cord presses thefirst set of braided fibers into the first groove.
 13. The reinforcedcore structure of claim 12, wherein a second groove is located at thesecond location and further comprising: a second cord tying off thesecond set of braided fibers and extending between the overlapped layersof the second set of braided fibers and opposite the second groove sothat the second cord presses the second set of braided fibers into thesecond groove.
 14. The reinforced core structure of claim 13, whereinthe perimeter of the expanded core between the first and second groovesis substantially the same as the perimeter of the expanded core in theregion between the first groove and the end and the overlapped layers ofthe first set of braided fibers extending over this latter area.
 15. Thereinforced core structure of claim 14, further comprising: a third setof braided fibers extending from the first end, past the first andsecond grooves, to a third groove beyond the second groove and reversingat the third groove over itself and back to the first end.
 16. Thereinforced core structure of claim 15, further comprising: a third cordtying off the third set of braided fibers and extending between theoverlapped layers of the third set of braided fibers and opposite thethird groove so that the third cord presses the third set of braidedfibers into the third groove.
 17. The reinforced core structure of claim15, wherein the perimeter of the expanded core between the first andsecond grooves and the overlapped layers of the second set of braidedfibers extending thereover is substantially the same as the perimeter ofthe expanded core in the region between the second and third grooves.18. The reinforced core structure of claim 13, further comprising: athird set of braided fibers extending from the first end, past the firstand second locations, to a third location beyond the second location andreversing at the third location over itself and back to the first end.19. A method of forming a reinforced core structure for use in a resintransfer molding process comprising: (a) providing an expanded corehaving a longitudinal axis; (b) braiding a first set of fibers from afirst end of the expanded core to a first location on the expanded core;(c) reversing the direction of the braiding of the first set of fibersat the first location and continuing braiding back to the first end sothat the first set of braided fibers is braided back upon itself to forma first dual layer fiber structure; (d) braiding a second set of fibersover the first set of braided fibers from the first end beyond the firstlocation to a second location; and (e) reversing the braiding directionof the second set of fibers at the second location back toward the firstend so that the second set of braided fibers is braided back upon itselfto form a second dual layer fiber structure.
 20. The method of claim 19,further comprising the step of tying the first set of braided fibers atthe first location with a cord before reversing direction of the braidedfibers.
 21. The method of claim 20, further comprising the step of tyingthe second set of braided fibers at the second location with a cordbefore reversing direction of the braided fibers.
 22. The method ofclaim 19, wherein the expanded core comprises a plurality of groovesextending transverse to the longitudinal axis;
 23. The method of claim22, wherein a first groove is located at the first location, and furthercomprising: tying the first set of braided fibers with a cord beforereversing direction of the first set of braided fibers, the cord beingarranged opposite the first groove such as to pull the first set ofbraided fibers into the first groove.
 24. The method of claim 23,wherein a second groove is located at the second location, and furthercomprising: tying the second set of braided fibers with a cord beforereversing direction of the second set of braided fibers, the cord beingarranged opposite the groove such as to pull the second set of braidedfibers into the second groove.
 25. The method of claim 24, wherein theperimeter of the expanded core between the first and second grooves issubstantially the same as the perimeter of the expanded core in theregion between the first groove and the end and the overlapped layers ofthe first set of braided fibers extending over this latter area.
 26. Themethod of claim 25, further comprising: braiding a third set of fibersfrom the first end over the first and second sets of braided fibers tobeyond the second groove to a third groove; and reversing the braidingdirection of the third set of fibers at the third groove back toward thefirst end so that the third set of braided fibers is braided back uponitself to form a third dual layer fiber structure.
 27. The method ofclaim 26 further comprising: tying the third set of braided fibers witha cord before reversing direction of the third set of braided fibers,the cord being arranged opposite the groove such as to pull the thirdset of braided fibers into the third groove.
 28. The method of claim 26,wherein the perimeter of the expanded core between the first and secondgrooves and the overlapped layers of the second set of braided fibersextending thereover is substantially the same as the perimeter of theexpanded core in the region between the second and third grooves. 29.The method of claim 19, further comprising: braiding a third set offibers from the first end over the first and second sets of braidedfibers to beyond the second location to a third location; and reversingthe braiding direction of the third set of fibers at the third locationback toward the first end so that the third set of braided fibers isbraided back upon itself to form a third dual layer fiber structure. 30.A method of preparing a reinforced core structure for a product to beformed in a resin transfer molding process utilizing a resin, the methodcomprising: (a) applying fibers over a core beyond the final finishedline for the product to be formed; (b) applying a tackifier solution tothe fibers located at the final finish line, the tackifier solutioncomprising a reduced resin concentration from the final resinconcentration of the product to be formed in the resin transfer moldingprocess; (c) locally consolidating the tackifier solution; and (d)cutting along the final finish line.
 31. The method of claim 30, whereinthe tackifier solution comprises resin to be used for the resin transfermolding process diluted by a solvent.
 32. A radius filler for use in aresin transfer molding system, the radius filler comprising: (a)unidirectional tows; and (b) a braided sleeve of fibers extending aroundthe unidirectional tows.
 33. The radius filler of claim 32, furthercomprising a tackifier solution added to the braided sleeve, thetackifier solution comprising a diluted mixture of the resin to be usedin the resin transfer molding system.
 34. The radius filler of claim 33,wherein the tackifier solution comprises resin to be used for the resintransfer molding process diluted by a solvent.
 35. A method of forming aradius filler for use in forming a preform to be used in a resintransfer molding process, the method comprising: (a) providingunidirectional tows; and (b) braiding a sleeve of fibers around theunidirectional tows.
 36. The method of claim 35, further comprising: (a)applying a tackifier to the braided sleeve, the tackifier comprising adiluted solution including the resin to be used in the final resintransfer molding process; and (b) consolidating the tackifier so as tolend rigidity to the radius filler.
 37. The method of claim 36, furthercomprising: (a) adjoining the radius filler with a preform; and (b)resin transfer molding the radius filler and the preform.
 38. A methodof forming a core structure comprising: (a) providing a mold having aninternal cavity; (b) arranging a prepreg along the inside of theinternal cavity, the prepreg being of a size such that the prepreg canextend around a circumference of the mold; (c) placing an expandablefoam material in the cavity of the mold and within the prepreg material;(d) heating the expandable foam material so as to expand the foammaterial within the prepreg material so to press the prepreg materialagainst the walls of the cavity of the mold; and (e) curing theexpandable foam material and the prepreg material so as to form the corestructure.